Mechanics
Fleet speed and fuel
How long a flight takes, what makes it faster and how much deuterium it burns.
A fleet flies at the speed of its slowest ship. How long a flight takes depends on that speed, the distance, the speed setting you pick when you send it and the universe's fleet speed, 1x here.
Distance
Flights inside one system are the shortest, flights to another system grow with the number of systems crossed, and flights to another galaxy take longest (Galaxy and density map).
Speed
A ship's base speed comes from its drive: the Combustion Engine, the Impulse Engine or the Hyperspace Engine. Every level of that research makes it faster, and some ships move to a better drive once the research is high enough. Ships lists the base speeds.
Boosts and events that raise fleet speed divide the flight time; officers and other permanent bonuses shorten it (How bonuses combine). No flight takes less than 5 seconds.
Vanguard players can raise their fleet speed with a faction perk (Factions).
Fuel
Every ship burns deuterium for the trip: more for a longer distance and for a higher speed setting. The fuel leaves the planet with the fleet, so keep deuterium in stock before you send one. Ships lists each ship's fuel use.
Fleet slots
Every fleet in flight takes a fleet slot, and Computer Technology gives you more of them.
Questions
Why does my fleet fly slower than its fastest ship?
A fleet flies at the speed of its slowest ship. Send fast ships on their own when time matters.
Does flying slower save fuel?
Yes. A lower speed setting on the send page makes the flight longer and burns less deuterium.